1. Departamento de Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Módulo 13, Campus de Excelencia UAM-CSIC, Cantoblanco, Madrid, 28049, Spain
Abstract:
The electronic structure of the two most stable isomers of squaric acid and their complexes with BeH2 were investigated at the B3LYP/6-311?+?G(3df,2p)// B3LYP/6-31?+?G(d,p) level of theory. Squaric acid forms rather strong beryllium bonds with BeH2, with binding energies of the order of 60 kJ?mol?1. The preferential sites for BeH2 attachment are the carbonyl oxygen atoms, but the global minima of the potential energy surfaces of both EZ and ZZ isomers are extra-stabilized through the formation of a BeH···HO dihydrogen bond. More importantly, analysis of the electron density of these complexes shows the existence of significant cooperative effects between the beryllium bond and the dihydrogen bond, with both becoming significantly reinforced. The charge transfer involved in the formation of the beryllium bond induces a significant electron density redistribution within the squaric acid subunit, affecting not only the carbonyl group interacting with the BeH2 moiety but significantly increasing the electron delocalization within the four membered ring. Accordingly the intrinsic properties of squaric acid become perturbed, as reflected in its ability to self-associate.
Figure
The ability of squaric acid to self-associate is significantly enhanced when this molecule forms beryllium bonds with BeH2